Java 集合系列（二）ArrayList详解

Java 集合系列（二）ArrayList详解

ArrayList

ArrayList 是通过一个数组来实现的，因此它是在连续的存储位置存放对象的引用，只不过它比 Array 更智能，能够根据集合长度进行自动扩容。

假设让我们来实现一个简单的能够自动扩容的数组，我们最容易想到的点就是：

add()的时候需要判断当前数组size+1是否等于此时定义的数组大小；

若小于直接添加即可；否则，需要先扩容再进行添加。

实际上，ArrayList的内部实现原理也是这样子，我http://们可以来研究分析一下ArrayList的源码

add(E e) 源码分析

/**

* Appends the specified element to the end of this list.

*

* @param e element to be appended to this list

* @return true (as specified by {@link Collection#add})

*/

public boolean add(E e) {

ensureCapacityInternal(size + 1); // 进行扩容校验

elementData[size++] = e; // 将值添加到数组后面，并将 size+1

return true;

}

/**

* The array buffer into which the elements of the ArrayList are stored.

* The capacity of the ArrayList is the length of this array buffer. Any

* empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA

* will be expanded to DEFAULT_CAPACITY when the first element is added.

*/

transient Object[] elementData; // non-private to simplify nested class access

private void ensureCapacityInternal(int minCapacity) {

ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); // elementData 数组

}

/**

* Default initial capacity.

*/

private static final int DEFAULT_CAPACITY = 10;

/**

* Shared empty array instance used for default sized empty instances. We

* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when

* first element is added.

*/

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

// 返回最大的 index

private static int calculateCapacity(Object[] elementData, int minCapacity) {

if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { // 与空数组实例对比

return Math.max(DEFAULT_CAPACITY, minCapacity);

}

return minCapacity;

}

private void ensureExplicitCapacity(int minCapacity) {

modCount++;

// overflow-conscious code

if (minCapacity - elementData.length > 0)

grow(minCapacity);

}

扩容调用方法，实际也就是数组复制的过程

/**

* Increases the capacity to ensure that it can hold at least the

* number of elements specified by the minimum capacity argument.

*

* @param minCapacity the desired minimum capacity

*/

private void grow(int minCapacity) {

// overflow-conscious code

int oldCapacity = elementData.length;

int newCapacity = oldCapacity + (oldCapacity >> 1);

if (newCapacity - minCapacity < 0)

newCapacity = minCapacity;

if (newCapacity - MAX_ARRAY_SIZE > 0)

newCapacity = hugeCapacity(minCapacity);

// minCapacity is usually close to size, so this is a win:

elementData = Arrays.copyOf(elementData, newCapacity);

}

add(int index, E element) 源码分析

/**

* Inserts the specified element at the specified position in this

* list. Shifts the element currently at that position (if any) and

* any subsequent elements to the right (adds one to their indices).

*

* @param index index at which the specified element is to be inserted

* @param element element to be inserted

* @throws IndexOutOfBoundsException {@inheritDoc}

*/

public void add(int index, E element) {

rangeCheckForAdd(index); // 校验index是否超过当前定义的数组大小范围，超过则抛出 IndexOutOfBoundsException

ensureCapacityInternal(size + 1); // Increments modCount!!

System.arraycopy(elementData, index, elementData, index + 1,

size - index); // 复制，向后移动

elementData[index] = element;

size++;

}

/**

* A version of rangeCheck used by add and addAll.

*/

private void rangeCheckForAdd(int index) {

if (index > size || index < 0)

throw neZNeipkRGLPw IndexOutOfBoundsException(outOfBoundsMsg(index));

}

从上面的源码分析可知，扩容和随机插入元素的消耗比较大，因此在实际开发中，应尽量指定ArrayList大小，减少在随机插入操作。

优缺点

优点

封装了一个动态再分配的对象数组

使用索引进行随机访问效率高

缺陷

在数组中增删一个元素，所有元素都要往后往前移动，效率低下

知识脑图

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